1. Field of the Invention
The present invention relates to computer graphics. In particular the invention relates to real time rendering of extremely detailed smooth surfaces with view-dependent tessellation using an improved level of detail approach. The invention utilizes a quad-tree map and geometric boundaries consisting of manifold non-self-intersecting surfaces.
2. Description of the Related Art
Swift advances in hardware, in particular faster, larger, and cheaper memories, have been transforming revolutionary approaches in computer graphics into reality. One typical example is the revolution of raster graphics that took place in the seventies, when hardware innovations enabled the transition from vector graphics to raster graphics. Another example which has a similar potential is currently shaping up in the field of surface rendering of volumetric bodies. This trend is rooted in the extensive research and development effort in visual graphics in general and in applications using real-time surface visualization, such as video games, terrain imaged interactive road maps and topographical maps for the aerospace industry.
Iso-surfacing algorithms can be classified as either view-dependent or view-independent. View-independent approaches in general generate geometry near the iso-surface. Most methods use triangles to approximate an iso-surface. The use of interval trees and the span space domain decomposition can greatly decrease the amount of work necessary to identify cells intersected by an iso-surface (also called active-cells), a major bottleneck in the extraction process. One advantage of generating geometry is that extraction need not be performed for each view. However, storing geometry becomes a burden as data resolution increases. View-dependent approaches focus on the resulting image and therefore attempt to perform computation mainly in regions that contribute substantially to the final image. View-dependent approaches are attractive in general as no intermediate form of the iso-surface needs to be stored explicitly, which greatly decreases storage requirements. One drawback of view-dependent approaches is that each time a new view is specified; the iso-surface extraction process must be repeated. For interactive applications, where viewing parameters are being changed frequently, such methods perform a relatively large number of computations. View-dependent approaches often offer excellent image quality, but frequently no geometric representation of the iso-surface is generated, making them undesirable for use in geometric modeling applications, for example. Dual contouring methods were introduced to preserve sharp features and to alleviate storage requirements by reducing triangle count.
Conventionally, a hierarchical data structure for describing an image, which is made up of a plurality of kinds of regions A, B, C, D and E, consists of a four or eight branch tree structure (so-called quad-trees and oct-trees). According to this system, the image is equally divided (decomposed) into four regions, and each region is recursively and equally subdivided (decomposed) into four sub regions until each sub region is made up solely of a single kind of region. The image data storage efficiency of this method is satisfactory, and the method enables basic image processing in the data structure. In addition, the image can be described in levels of rough steps to fine steps. However, there is a problem in that the number of nodes increases especially at boundary portions of the data structure. In the four-branch tree structure, three nodes and a leaf branch out from a root node. The node is indicated by a circular mark and corresponds to the region or sub region made up of two or more kinds of regions. On the other hand, the leaf is indicated by a black circular mark and corresponds to the region or sub region made up solely of a single kind of region.
Frequently, objects (such as, for example, characters in a video game or terrain in a virtual roadmap) are generated using a so-called “base mesh” composed of a minimum number of large polygons, and so provides a minimum level of rendering detail. The polygons forming the base mesh are normally referred to as “primitives”. These primitives are normally selected to enable the position and orientation of the object within a scene to be rapidly (and unambiguously) defined, and thereby facilitate appropriate scaling and animation of the object.
The process of defining polygons within a primitive is referred to as “tessellation”, and the number of polygons defined within a primitive is given by the “tessellation rate”. Formally, the tessellation rate is the number of segments into which an edge of the primitive is divided by the vertices of the polygons defined within the primitive. Thus, for example, a primitive has (by definition) a tessellation rate of 1. If one or more polygons are then defined within the primitive so one new vertex lies on each edge of the primitive (thereby dividing each edge into two segments), then the tessellation rate will become 2. Similarly, If other polygons are then defined within the primitive such that two vertices lie on each edge of the primitive (thereby dividing each edge into three segments), then the tessellation rate will be 3. As may be appreciated, since the tessellation rate (or value) is based on the number of segments into which an edge of the primitive is divided, the tessellation rate can be defined on a “per-edge” basis. In principle, this means that different edges of a primitive may have the same, or different, tessellation values.
In cases where a higher level of detail is required, additional polygons can be defined within each primitive, as needed.
There are a variety of existing methods that aim at reducing the amount of geometric primitives that are processed by the rendering pipeline proper. One general technique, called occlusion culling, operates by eliminating sections of the geometry that are invisible from the current view port (or from any of its immediate surrounding area or volume). Another technique uses several refinement levels for the geometry. Then the system renders only the crudest representation of geometry that will result in less than a certain acceptable level of visible error when compared against an image rendered from the exact geometry. This approach is known as a “level-of-detail scheme” in the art. The present invention improves on this category, in particular utilizing the quad-tree-based subdivision approach. The present invention has considerably less requirements on the structure of the geometry than state-of-the-art methods based on this approach, yet still benefiting from its simplicity and efficiency. These weaker requirements allow the herein inventive method to use geometric representations that have significantly less geometric primitives than is typical. This means the system will have to render considerably less primitives in real time than is used by methods known in the art to achieve the same level of fidelity. The present invention advantages are especially prevalent when rendering such level of fidelity on limited computing devices in terms of processing power and memory allocation as well as when streaming the geometric primitives over limited bandwidth communication.